American Oil and Gas Reporter - March 2017 - 81

SpecialReport: Midstream & Infrastructure Update
Gas Engine Emissions
Engine manufacturers and retrofitters
are looking for ways to minimize the cost
of emissions compliance, assures Rick
Fisher, vice president of sales for Continental Controls Corp. He says achieving
that goal requires a keen understanding of
the factors that contribute to emissions.
"For natural gas engines, emissions
contributors include changes in engine
load, engine and catalyst temperatures,
fuel composition, as well as drifting sensors and catalyst deactivation," Fisher
outlines. "Today, the industry has solutions
for all of these contributors."
To illustrate, Fisher points to catalyst
deactivation. "As the catalyst on an engine
ages, it deactivates and becomes less effective," he notes. "Using larger catalysts
can extend how long it takes for the deactivation to occur, but eventually, even
the largest catalyst can become masked
or contaminated."
Even a healthy catalyst can have problems with oxygen storage if the air/fuel
ratio controller keeps the oxygen sensor's
set point steady, Fisher warns. When that
happens, he says the catalyst can belch
carbon monoxide to reset itself, sometimes
compromising the engine emissions' compliance.
"To keep that from happening, engines
should vary the air-to-fuel ratio from rich
to lean and back again rapidly, a technique
called 'dithering,'" Fisher says. "The constant changes scrub the catalyst to keep
it active and improve its oxygen storage
capacity."
Fisher says modern engines must respond quickly to changes in engine load,
engine temperature and gas composition,
particularly if the engine is running on
lower-quality field gas. Toward that end,
he recommends using a full-authority
fuel valve. "A full-authority valve acts as
the last pressure regulator, rather than
merely trimming the fuel, which gives it
much greater control over the amount of
fuel that enters the combustion chamber,"
he explains.
To maximize that control, Fisher suggests looking for a valve that can open
and close quickly. "With the right design,
a valve can go from one end to the other
in 30 milliseconds," he says.
Ideally, Fisher says the engine should
mix air and fuel using a venturi rather
than a carburetor. "Venturis improve
air/fuel mixing by creating a more homogenous mix and distributing fuel more

Continental Controls Corp. recommends using a venturi rather than a carburetor to mix
air and fuel inside natural gas engines. In addition to distributing fuel more evenly, the
company says a properly designed venturi can maintain a given air-to-fuel ratio during
load changes, enabling the engine to keep the ratio that minimizes emissions. Continental
Controls adds that a venturi also can be used in a system for converting a diesel engine
to run in part on natural gas, an upgrade that saves money while reducing emissions.

evenly to all cylinders," he explains.
"Many traditional carburetors, were designed to run richer at higher loads. Venturis that are designed correctly can keep
the air/fuel ratio the same at all loads, allowing the AFR control to control more
precisely during load changes."
Fisher says a venturi also should be
designed to minimize pressure drop. "Every
venturi will cause some pressure drop,
but if the back end is shaped appropriately,
it is possible to recover 70-80 percent of
it," he reports. "This keeps supply pressures
and the associated costs low."
To determine the best air/fuel ratio,
Fisher recommends a multiloop control
scheme that Continental Controls developed to meet California's notoriously
strict emissions limits with minimal human
intervention. Like many systems, the
company's approach involves monitoring
position, pressure and oxygen. However,
it adjusts the set point for the oxygen
sensor by monitoring a NOx sensor on
the outside of the catalyst.
With a carefully designed algorithm,
Fisher says the control system can use
the NOx sensor to find the ideal balance
between rich and lean operation. "When
an engine runs too lean, it makes too
much NOx. In theory, if we tried to minimize NOx emissions, the engine would
run richer and richer. However, the richer
it gets, the more ammonia it produces.

"The ammonia appears as NOx to the
sensor," Fisher says. "By trying to minimize NOx as calculated by the algorithm,
the system finds the oxygen sensor set
point that puts the engine in the optimum
position to optimize the nonselective catalyst and maximize emissions reduction."
In many jurisdictions, Fisher notes
that engine operators are required to monitor and record parameters such as the
temperature at the catalyst inlet and the
differential pressure across the catalyst.
"With time, such requirements will become
more common," he predicts.
As prices and activity have improved,
Fisher says Continental Controls has
gotten more calls from drilling contractors
interested in using its gas substitution
system to enable diesel engines to run on
a mixture of diesel and natural gas. "In
many applications, the system can achieve
a gas substitution rate around 70 percent,"
he reports. "Because natural gas is cheaper
than diesel, the system often can pay for
itself within a year. If the differential between diesel and natural gas prices widens,
as it has in the past, the payback period
will be even faster."
Integrated Systems
Since the start of 2017, FW Murphy
Production Controls has seen orders for
emissions-related equipment increase,
says Darrell Schmitt, the company's techMARCH 2017 81

American Oil and Gas Reporter - March 2017

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